The Redis health checker is a custom health checker (with envoy.health_checkers.redis as name)
which checks Redis upstream hosts. It sends a Redis PING command and expect a PONG response. The upstream
Redis server can respond with anything other than PONG to cause an immediate active health check failure.
Optionally, Envoy can perform EXISTS on a user-specified key. If the key does not exist it is considered a
passing health check. This allows the user to mark a Redis instance for maintenance by setting the
specified key to any value and waiting
for traffic to drain.